Walker expressed confidence he would hold on to his seat shortly after the Government Accountability Board ordered the election, after more than 900,000 signatures were collected supporting a recall in the wake of Walker’s push against union bargaining rights. It marks only the third recall of a governor in U.S. history.

Milwaukee Mayor Tom Barrett announced hours later he would challenge Walker, shaking up a Democratic primary race that had been led by union-backed candidate Kathleen Falk. Barrett has publically clashed with unions who were urging him not to get into the race.

In an email to supporters, Barrett said he would begin campaigning immediately to win the primary that looms just 39 days away on May 8. The general election is June 5.

Hours earlier, Walker said he was ready to defend his record just 15 months into his tumultuous term.

“My hope is just as we earned the trust of the majority of voters in November 2010 that we’ll have a chance to earn that trust again this June,” Walker said before touring a Milwaukee manufacturing plant. “So I look forward to the opportunity to share that message.”

Walker’s campaign spokeswoman branded Barrett as a two-time loser who failed to turn around Milwaukee’s economy, noting that he also lost in the Democratic primary for governor in 2002.

But Barrett said Walker hasn’t lived up to his promises.

“Scott Walker came into the governor’s office promising to create 250,000 jobs, and to bring us together,” Barrett wrote in the email. “Instead, he divided our state like never before and presided over a Wisconsin economy that last year lost more jobs than any state in the country.”

The election board’s 5-0 vote that ordered the election came Friday morning, though the decision had been expected for weeks. The board had already determined there were 900,939 valid signatures on petitions supporting the recall, though only 540,208 were required. The board also ordered recalls for Lt. Gov. Rebecca Kleefisch and four Republican state senators.

Walker was targeted for recall after he pushed through a law last year that effectively ended collective bargaining rights for most state workers. It also forced the workers to contribute more to their pension and health care costs, which amounted to a cut in pay.

Walker argued the changes were needed to help balance the state’s budget, while Democrats and other opponents said the true intention was to weaken the power of unions, which have traditionally opposed Republicans.

A passionate fight ensued in Wisconsin, the first state to enact a comprehensive collective bargaining law in 1959 and the birthplace of the national union representing all non-federal public employees. Protests raged for weeks and grew as large as 100,000 people. But Walker and Republicans who controlled the Legislature never wavered and they passed the law even though all 14 Senate Democrats fled to Illinois in a failed attempt to block it.

Barrett said he would fight to restore collective bargaining rights “because it’s the right thing to do, and it’s necessary to heal Wisconsin.” Falk, the former Dane County executive, won the support of unions after she promised to veto any state budget that doesn’t restore collective bargaining, a pledge Barrett refused to make.

Falk issued a statement welcoming Barrett into the race. Two other lesser-known Democrats, Secretary of State Doug La Follette and state Sen. Kathleen Vinehout, are also running.

Two Republican state senators lost their seats in a previous round of recalls last year. The Senate is currently evenly divided between Republicans and Democrats. Republicans control the Assembly 59-39-1.

The Wisconsin Republican Party said Friday that it plans to force primaries in each of the four Senate races by renewing a tactic it used last year: running a fake Democratic candidate in each race.

Stephan Thompson, the state party’s executive director, said the move guarantees one clear date for both the primary and general elections. That prevents Democrats from trying to hold the elections for state Senate on a different day from the governor and lieutenant governor races, he said.

Zac Kramer, the executive director of the State Senate Democratic Committee, criticized the ploy, saying local voters were “sick of these blatant political games.”

The recall elections promise to be bare-knuckle expensive affairs, with millions of dollars coming into Wisconsin from out-of-state special interests. Walker already had raised more than $12 million by mid-January and he has traveled across the country getting checks as large as $250,000 from high-powered conservative backers.

An outside group supported by unions has run ads supporting Falk. Walker has been on the air since mid-November and the Republican Governors Association started ads earlier this month attacking both Falk and Barrett. And the RGA launched two new ads Friday, one against Barrett and one against Falk.

At least three Democrats also have announced their plans to run for lieutenant governor. Candidates wishing to run in any of the recalls have until April 10 to file nomination papers.

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Associated Press writer Dinesh Ramde contributed to this story from Milwaukee.